Certain muscular dystrophies preferentially affect a specific skeletal muscle fiber type, either fast or slow twitch. In particular, fast muscle fibers re more susceptible to damage than slow fibers in human and mouse models of Duchenne Muscular Dystrophy (DMD). In the mouse model, many genetic and epigenetic factors can ameliorate the effects of DMD, possibly by promoting a slow muscle phenotype. However, it is not clear from these studies whether fiber type identity is indeed essential for conferring susceptibility or resistance to muscular dystrophy. This project is aimed at an ultimate goal of manipulating skeletal muscle fiber type as a treatment for muscular dystrophy. The objectives of this proposal are to systematically test whether genetic and epigenetic regulators of fiber-type identity can modulate the muscular dystrophy phenotype and to identify new epigenetic molecules that regulate fiber-type differentiation. This proposal will directly test a fundamental hypothesis: that factors that promote slow muscle differentiation will ameliorate the effects of DMD. Recent studies, including work from our lab, have identified critical genetic and epigenetic mechanisms that regulate fiber-type identity. These studies now create an opportunity to directly test whether fiber-type modulation is a viable therapeutic approach for muscular dystrophies and to identify candidate molecules that will provide the basis for therapeutic studies in zebrafish and, ultimately, in mammals. This proposal will take advantage of zebrafish models to address two Specific Aims. Aim 1 will directly test whether muscle fiber type confers susceptibility or resistance to muscular dystrophy by determining whether factors that regulate fiber-type differentiation enhance or suppress the zebrafish dmd phenotype. We will initiate our approach by manipulating fiber-type regulators that function early in development in zebrafish embryos lacking Dystrophin. Aim 2 will identify new epigenetic factors that regulate muscle fiber type by screening an epigenetic chemical library using live trans- genic zebrafish that co-express fluorescent reporters for fast and slow muscle. The epigenetic chemicals will also be screened for their abilities to enhance or suppress the zebrafish dmd phenotype. This project will thus identify genetic and epigenetic regulators of muscle fiber-type identities that confer susceptibility or resistance to muscular dystrophy. These expected outcomes are significant because the genetic and epigenetic regulators identified by this proposal are highly likely to provide new targets for therapeutic studies. The recent success of epigenetic chemical screening in other developmental and disease processes highlights the exceptional opportunity and innovation such screening will provide for the skeletal muscle differentiation and muscular dystrophy fields.